Pulse interval = round-trip time + post-echo delay

Steps:

• Round-trip distance: 2 × 6000 m = 12000 m (noting likely typo for 600 m to match options).
• Round-trip time: 12000 m / 1500 m/s = 8 s.
• Add delay after receiving echo: 8 s + 2.0 s = 10 s.
• Thus, time from one pulse to the next is 10 s.

Why D is correct:

• The interval equals round-trip time (2 × depth / speed) plus the 2.0 s delay after echo reception, per sonar timing procedure.

Why the others are wrong:

• A: Assumes round-trip time of 2 s + 2 s delay, underestimating distance or overestimating speed.
• B: Assumes round-trip time of 4 s + 2 s delay, possible if depth halved.
• C: Assumes round-trip time of 6 s + 2 s delay, possible if speed reduced to ~1000 m/s.

Final answer: D